It has been determined that heat weldable thermo-formed plastic, and in particular high density polyethylene "HDPE" is the preferred material for secondary containment piping systems for hazardous wastes. HDPE is virtually inert to most hazardous wastes being disposed of and has been accepted as the required material for both the inner carrier pipe as well as the external containment pipe.
In such systems there is a requirement for joining sections of pipe together to like sections and or to appropriate fittings, to create DWV or sewer reline plumbing systems for handling these hazardous effluents.
Butts et al, United States patent application Ser. No. 07/941,662 filed Sep. 8, 1992 (U.S. Pat. No. 5,407,514) the entire subject matter of which is herein incorporated by reference, discloses an improved form of welding rod which can be used to weld members of thermoplastic. This welding rod when used for HDPE members consists of an HDPE core wrapped in a plurality of fine wires, which are embedded in the surface of the core. The welding rod is placed between the members to be joined, pressure is applied and maintained while an electric current is passed through the wires causing melting of the core and the adjacent portions of the members and the pressure is maintained after the current is discontinued until the members are fused together. The said prior patent application also discloses a variety of techniques for welding pipes. These techniques include positioning an annulus of welding rod between the ends to be joined and butting the ends together with the appropriate maintenance of pressure while an electric current is supplied for a sufficient time to cause fusion of the members and the welding rod.
The said application also discloses the use of a fusion ring for forming butt welds between sections of pipe, the fusion ring incorporating an appropriate welding rod element internally, with the ends of the pipe being received in opposite sides of the ring and held in position while the welding rod is heated and fuses with the ends of the pipe to form the completed joint. This fusion ring is formed of a welding rod and a collar surrounding the welding rod, the collar having an inside diameter substantially equal to the outside diameter of the sections to be joined. The fusion ring is positioned between the two sections of pipe as in a socket fitting and the two sections are then welded together while pressure is applied to push the pipes together. The use of the collar is particularly advantageous in situations where containment of the fluid transported in the pipe is a priority. For larger diameter and or thicker walled pipes multiple runs of the appropriate welding rod can be utilized in the collar.